In many direct current (DC) coupled amplifiers, which may include several stages, negative voltage feedback from the output to the input is used to stabilize the DC operating point such that all signal nodes remain within the linear operating range of the amplifier. A resistor-capacitor (RC) low pass filter or transconductance amplifier-capacitive integrator (in which each circuit uses a grounded capacitor) is placed in the feedback path to attenuate the loop gain at higher frequencies so that the amplifier is stable and the gain of the amplifier in the pass band is not degraded.
In some relatively low frequency applications, such as the base band filter/amplifier for a zero-intermediate frequency (zero-IF) receiver for example, it is desirable to implement the entire circuit on an integrated circuit. This reduces external parts and integrated circuit (IC) pinouts, and improves tracking between the I and Q channels of the zero-IF receiver. However, RC low pass filters can require a large amount of chip area to implement at low frequencies, which can increase chip cost. Chip area is not as much a problem with the transconductance amplifier-capacitive integrator approach since the gain of the transconductance amplifier can be lowered as necessary to achieve the desired loop corner frequency.
Both approaches suffer from a warm-up time problem in applications where the circuit is strobed ON and OFF to improve battery life, due to the need to charge the grounded capacitor to the proper DC operating point following each power-up. It is therefore desirable to provide a differential DC offset compensated amplifier circuit having a fast warm-up time in a completely integrated configuration that minimizes external parts and IC pinouts. It is additionally desirable to provide an amplifier circuit which has the capability to externally program a high pass corner frequency generated by the differential feedback loop while maintaining a fast warm-up time.